1. Field of the Invention
This invention generally relates to a charging device for charging a residual toner left on an image carrier after an image transfer operation by means of a first charging member and collecting the charged residual toner by means of a second charging member for charging the image carrier, an image forming apparatus, or an electronic photographic apparatus, such as an electrostatic copying machine, a printer, or a facsimile machine and a method of charging an image carrier of an image forming apparatus.
2. Description of the Related Art
JP-A-2005-331846 (to be referred to as Document 1 hereinafter) and JP-A-2004-12582 (to be referred to as Document 2 hereinafter) propose an image forming apparatus having a charging device prepared by combining first and second charging members for charging an image carrier where an electrostatic latent image and a toner image are formed.
An image forming apparatus described in Document 1 is designed to remove the residual toner adhering to a photosensitive member after an image transfer operation by scraping it off by means of a photosensitive member cleaning brush. However, the toner, if of a small quantity, that is not scraped off from the photosensitive member by the photosensitive member cleaning brush then adheres to a charging roller, which is a second charging member for electrically charging the photosensitive member, to contaminate the charging roller. Then, it is difficult to desirably charge the photosensitive member. To avoid this problem, the polarity of the bias applied to the charging roller for the purpose of forming an image is subsequently inverted in order to remove the contaminant adhering to the charging roller, which is a second charging member, in the image forming apparatus described in Document 1. With this arrangement, the surface potential of the photosensitive member can be stabilized for a long period of time so that high quality images can be obtained without disturbances.
When, however, toner containing an external additive to a high ratio is employed in such an image forming apparatus, the external additive can easily escape the cleaning blade and remain on the charging roller. The external additive does not escape uniformly in the longitudinal direction of the cleaning blade. In other words, it may escape to a large extent in some local areas and but only to a small extent in some other local areas. In the local areas where the external additive escapes the cleaning blade to a large extent, the above-described technique of applying a bias of the opposite polarity to the charging roller does not provide an effect of sufficiently cleaning the photosensitive member and can unevenly smear the charging roller, which is a second charging member. Thus, it is then difficult to satisfactorily charge the photosensitive member by means of the second charging member and the photosensitive member can be defectively charged.
The first charging member of an image forming apparatus described in Document 2 is realized by using a brush roller and the apparatus is designed to remove the toner showing a positive polarity or the toner showing the opposite polarity and adhering to the photosensitive member after a transfer operation and the discharge products that are made to adhere to the photosensitive member when the photosensitive member is charged by the charging roller, which is a second charging member, in order to prevent those foreign objects from adhering to the surface of the charging roller of the photosensitive member. With this arrangement, the surface potential of the photosensitive member can be stabilized for a long period of time so that high quality images can be obtained without disturbances.
However, the brush roller, or the first charging roller, of an image forming apparatus described in Document 2 is designed to temporarily seize the foreign objects from the photosensitive member. Thus, as the quantity of the foreign objects that are seized by the brush roller increases, they are transferred onto the photosensitive member again and removed by way of a transfer unit. Therefore, such foreign objects are deposited on the surface of the photosensitive member in the long run. The foreign objects that are adhering to the photosensitive member after the transfer include those that cannot be seized by the brush roller so that such foreign objects can be further deposited on the surface of the photosensitive member. Thus, it is then difficult to satisfactorily charge the photosensitive member by means of the charging roller, which is a second charging member, and the photosensitive member can be defectively charged.